The present invention relates to a process for the manufacture of bleached high-yield pulps.
High-yield pulps are manufactured by subjecting the starting lignocellulosic matter, such as wood in the form of chips, to an action of mechanical type, combined or otherwise with actions of chemical and thermal type.
High-yield pulps are of very high interest to industry, because they produce an acceptable compromise between solely mechanical pulps and chemical pulps.
"High-yield" means a yield weight of pulp in the dry state relative to the weight of the initial matter in the dry state, which is generally equal to at least 85% and frequently even to 90%.
Bleaching of high-yield pulps, which is made necessary by the quality which is required for paper and similar products in this field, is carried out with the aid of hydrogen peroxide in alkaline medium. Such bleaching also presents a problem both from the technical standpoint and the economic standpoint.
In fact, in order to reduce the loss of effectiveness of hydrogen peroxide, due to its instability in alkaline medium, sodium silicate is used in a quantity such that it is the cause of multiple and considerable difficulties during the production of pulp and that of paper, such as, for example, the formation of solid deposits on production of a paper with a rough feel. Reference can be made on this subject, for example, to C. W. Kutney, Pulp & Paper Canada, 86:12, (1985), 182-189.
Furthermore, the quantity of sodium silicate cannot be reduced without consequently affecting the result of bleaching, making the pulp unacceptable.
To ensure that this result remains sufficiently high, the matter to be bleached may also be subjected, for example, to two stages of bleaching with H.sub.2 O.sub.2 instead of one, or attempts are made to substitute the silicate with an agent of similar effectiveness, such as diethylenetriaminepentamethylenephosphonic acid (DTMPA), whose equivalence to sodium silicate is apparently verified only in special cases, according to Robert W. Allison, Appita, vol. 36, no. 5, March, 1983, 362-370. Such solutions are not satisfactory in all cases from the economic standpoint.